International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN (P): 2249-6890; ISSN (E): 2249-8001 Vol.9, Issue 6, Dec 2019, 13-24 © TJPRC Pvt. Ltd. STABILITY IMPROVEMENT IN NATURAL CIRCULATION LOOP USING TESLA VALVE – AN EXPERIMENTAL INVESTIGATION *U. C. ARUNACHALA 1, A. C. RAJAT 2, DHRUV SHAH 3 & UJJAWAL SUREKA 4 1,3,4 Department of Mechanical & Manufacturing Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India 2Department of Mechanical Engineering, National Institute of Technology Karnataka, India ABSTRACT Natural circulation loop works on the basic principle of thermosiphon effect, which doesn’t require any external force to drive the fluid in a loop. Hence they are highly applicable in power industries where there are a source and a sink. However, the flow pattern in a single-phase natural circulation loop is quite unstable especially the horizontal heater horizontal cooler configuration which results in lower loop effectiveness. A lot of research has been carried out to study these instabilities and find a way to curb them. Techniques such as tilting of loop, use of nanofluids, adding flow resistances etc. have shown promising results. But they have their own practical constraints. Hence in the present experimental study, a Tesla-type passive valve is incorporated in natural circulation loop. Due to its presence, not only Article Original stability period is reduced (at least 90 s compared to non-Tesla loop) but also unidirectional flow is achieved. The Tesla valve performed better for a broad range of heater power (90-170W) and was also able to provide noticeable resistance to the flow when the loop was inclined by 2.5° in the opposite direction. KEYWORDS: Natural Circulation Loop, Tesla Valve, Loop Stability & Horizontal Heater Horizontal Cooler System Received : Jul 22, 2019; Accepted : Aug 15, 2019; Published : Sep 28, 2019; Paper Id .: IJMPERDDEC20192 1. INTRODUCTION The term circulation refers to a closed-circuit carrying fluid that will be under motion. Circulation or flow can be of two types viz. natural and forced. The forced type is catered by pumps, blowers etc. based on the application and fluid to be transported. The natural circulation (NC) also referred to as thermosiphon or natural convection systems which do not require any physical component to trigger its motion but needs a channel or path for it to travel. The driving force for the flow occurs due to the presence of heat source and heat sink. This is the reason why natural circulation loop (NCL) finds application in power industries, solar water heaters, transformer cooling, geothermal power extraction, cooling of internal combustion engines, gas turbine blades and nuclear reactor cores. In heat pipe, the heating and cooling happen in the same leg/channel whereas, in NCL there are two separate legs in which fluid can take either direction once getting heated. NCL behaves differently for the various heater and cooler orientations as thoroughly researched by both numerically and experimentally. Horizontal heater horizontal cooler (HHHC) is the least stable arrangement (Figure 1) and has been researched a lot in order to improve its directional stability and reduce the time it takes to form stable flow. The fluid in the horizontal position can take either of the clockwise or anti-clockwise direction, giving rise to uncertainty and ineffectiveness. However, vertical heater vertical cooler (VHVC) is the most stable configuration as flow is readily established forming a unidirectional and stable loop. Other orientations are horizontal heater vertical cooler (HHVC) and vertical heater horizontal cooler (VHHC). www.tjprc.org SCOPUS Indexed Journal [email protected] 14 U. C. Arunachala, A. C. Rajat, Dhruv Shah & Ujjawal Sureka Due to the lack of control and interplay of various forces in NCL, various kinds of instability can arise. The flow is considered stable when the parameters like velocity and temperature do not change with time. All NC systems take a certain amount of time to stabilize. The alternating effects of buoyancy and gravity due to change in density across the loop causes such oscillation. It also includes the actions of friction and velocity. The flow in HHHC type loop may take any direction, and to have a more stable and controllable system, flow need to be unidirectional. 2. TESLA VALVE A Tesla valve is a valvular conduit, fixed-geometry passive check valve without moving parts. It allows fluid to flow preferentially in one direction. The device is named after Nikola Tesla, who was awarded a patent in 1920 for its invention [2]. To increase the efficiency of the Tesla valve, one has to increase the resistance in the reverse side and also have a lower pressure drop in the forward direction. In an attempt to obtain this, D-type Tesla valve is designed as shown in Figure 2. However the old version is depicted in Figure 3. 3. LITERATURE REVIEW In this section, the recent literature review related to single-phase HHHC type of NCL, instabilities and restraining techniques in NCL, and Tesla valve have been discussed. To understand the fluid behaviour, Misale et al. [4] have carried out experiments in a rectangular loop filled with water and water-glycerol mixture. It was found that the temperature variations throughout the loop followed a pattern of damped oscillations until it reached a steady state. The one-dimensional analytical procedure by Keller [5] for a loop indicated that the fluid took a particular direction and exhibited periodic motion in terms of its magnitude of velocity. These oscillations were observed to be mainly dependent on the frictional and buoyancy forces. Vijayan [6] has experimentally captured the characteristics of steady-state and stability behaviour of NCL. It was verified that the steady- state Re in uniform or non-uniform diameter NCL could be expressed as, Figure 1: NCL- HHHC System [1]. Figure 2: D-Type Tesla-Valve [3]. Figure 3: Cross-Section of a Tesla Valve [2]. Impact Factor (JCC): 8.8746 SCOPUS Indexed Journal NAAS Rating: 3.11 Stability Improvement in Natural Circulation Loop using 15 Tesla Valve – An Experimental Investigation r Re ss =C[Gr m/N G] (1) Where the constants r and C depend on the nature of the flow (laminar or turbulent), NG is the geometric ratio and Gr m is the modified Grashoff number. Further, the stability criteria for uniform and non-uniform diameter loop was defined as a function of Stanton number, modified Grashoff number and geometric ratio. The effect of wall friction on stability of NCL in different flow regimes was numerically studied by Ambrosini et al. [7] and concluded that the friction model to be used in a loop is very much dependent on the geometry. Swapnalee and Vijayan [8] have done analytical and experimental studies with respect to four different orientations of the heater and cooler and, presented general flow equations assuming partial laminar and partial turbulent flow. The obtained equations were in good agreement with the data in earlier literary works. Yadav et al. [9] have conducted steady-state numerical analysis in CO 2 (sub-critical/super-critical) based loop with end heat exchangers. It shows that if the average temperature of the loop increases, the mass flow rate and the heat transfer rate increases until the pseudo-critical point and then decreases. Further, correlations are also proposed to predict the Re and Nu values in case of NCL with end heat exchangers. In similar study, Yadav et al. [10] have performed experimental transient analysis at sub-critical and supercritical operating conditions (50-100 bar and 323-353K) by varying loop tilt angle and external mass flow rate. It was observed that the heat transfer rate increases with increase in operating temperatures and external mass flow rate, whereas it decreases with the loop tilt angle. Additionally, it was reported that the loop is quite stable under the influence of any disturbances inflected. Doganay and Turgut [11] studied the impact of input power, inclination and the condenser temperature on the thermal performance of a mini loop by varying the particle concentrations of Al 2O3-DIW (De-ionised water) nanofluid. As the concentration of the nanofluids and inclination increases, the thermal effectiveness of the system improves. It was also reported that Al 2O3-DIW nanofluid has enormous potential to overhaul the thermal management problems in large heat dissipation devices due to its better effectiveness in high-temperature conditions. Cheng et al. [12] have formulated 1-D mathematical model for loop with end heat exchangers. The formula was derived for temperature distributions along the loop. It was observed that the modified Grashoff number, heat transfer rate and Reynolds number increase with the increase in driven temperature difference. Further, Cheng et al. [13] have theoretically and experimentally studied the heat transfer and fluid flow characteristics of loop with double pipe heat exchangers. The cooling fluid temperature is fixed (10°C), and the heating fluid inlet temperature was varied from 30°C to 60°C. It was observed that stability reached for all the cases but the stabilizing time decreases with an increase in heating fluid inlet temperature. Upon using the Nu correlation for laminar flow, i.e. Nu=3.66, significant error between the theoretical and experimental results was observed. Hence a new correlation (Equation 2) was formulated which showed much better agreement. Nu=0.2702 Re 0.4713 (2) A lot of research highlights analyses in NCL [4-13]. However, analysis of HHHC type is critical as they showcase a twofold instability behaviour. Vijayan et al. [14] have discussed the stability of loop with different heater and condenser orientations and experimentally found VHVC and HHHC orientation as the most stable and least stable, respectively. Kudariyawar et al.